In the field of paint, reduction of the amount of organic solvent used is becoming an important issue from the viewpoints of environmental protection and conservation of resources. In recent years, the development of aqueous paints and paints having a high solid content has proceeded as a countermeasure for reducing the amounts of organic solvents in paints.
As aqueous paints in the field of automobile paint, there are known thermosetting aqueous paints containing polyvalent carboxylic acid resins, amino resins, linear low molecular weight polyester diols and benzoin as the main components [see, for example, Japanese Patent Application Laid-open No. Hei 4 (1992)-93374]. However, these aqueous paints have a low solid concentration, inadequate smoothness and so on of the paint film, and inadequate paint film performance with respect to water resistance of the paint film and so on.
In the case of automobile paint in particular, the overcoating paint is required to be a paint film having superior quality in terms of appearance, and during the application of metallic paint, a paint film appearance having a superior metallic feeling and so on is required to be obtained by satisfactorily orientating a scaly, effect pigment such as aluminum in a metallic base paint.
A proposed example of an aqueous metallic base paint used for such a metallic coating is a base coat composition containing crosslinked polymer fine particles in an aqueous base paint that is capable of maximally demonstrating flip-flop effects of the metallic pigment while also forming a paint film having a superior metallic appearance [see, for example, Japanese Patent Publication No. Hei 3(1991)-14869]. However, this composition has the shortcoming of being susceptible to the occurrence of dripping, unevenness and other paint film defects due to changes in painting conditions, and particularly humidity.
In addition, in the case of double-coating, single-baking painting methods, although methods have been proposed for forming a paint film of a paint used to apply an aqueous metallic base that uses an aqueous dispersion in which a specific long-chain monomer such as stearyl acrylate or stearyl methacrylate is copolymerized in an aqueous dispersion containing polymer fine particles (see, for example, Japanese Patent Application Laid-open No. 2001-104878), this had the problems of having inadequate painting workability, smoothness and metallic feeling.